The semiconductor device manufacturing industry has relied on optical lithography techniques since the 1960s to produce ever denser and more powerful integrated circuit (IC) chips. However, the ability to keep reducing the minimum geometry of IC chips is reaching fundamental material limitations. The continued use of optical lithography has been enabled by the employment of increasingly more complex lenses and shorter operating wavelengths. Present-day microlithography lenses rely on liquid immersion to increase the numerical aperture (NA) to a maximum of about 1.33.
Likewise, the optical wavelengths used to expose the photoresist have been reduced from the original g-line of mercury (436 nm) down to the ArF-excimer-laser deep-ultraviolet wavelength of 193 nm. Beyond 193 nm there are no glass materials suitable for use as the lenses, and the transmission of such short-wavelength light through air is problematic. While serious effort is presently being directed toward using extreme-ultra-violet (EUV) wavelengths in the X-ray region at 13.5 nm in combination with mirror systems, it is not yet clear whether EUV lithography systems will be commercially viable.
Optical lithography relies on the ability of photoresist to respond to light incident thereon and thereby record a sharp photoresist image. Conventional mask-based optical lithography is limited mainly by the diffraction limit of the projection imaging process.
U.S. Pat. No. 7,713,684 (hereinafter the '684 patent) describes a direct-write (i.e., non-mask-based, non-projection) optical-lithography technique whereby a thin film is placed above a photoresist layer. The thin film can be bleached by a first wavelength of light and rendered opaque by a second wavelength of light. The '684 patent describes a technique that involves creating a patch of the second wavelength, the center of which contains a small black hole. The hole is irradiated with an image larger than the hole and formed by the first wavelength of light, and if the intensity of the superimposed second wavelength of light is sufficiently low, then the first wavelength of light bleaches and thereby exposes the underlying photoresist layer.
The size of the resultant photoresist image can be as small as 1/13th the size of the image formed using just the first wavelength. This technique and subsequent related techniques whereby an image is formed in photoresist using direct writing to achieve a resolution beyond the usual resolution limits of conventional photolithography are referred to in the art as super-resolution lithography (SRL).
While SRL techniques have been demonstrated and are shown to be feasible, they need to be made commercially viable. This includes developing systems and methods that allow for SRL techniques to be implemented in a manufacturing environment in a manner that provides wafer throughputs similar to those presently available using conventional mask-based optical lithography techniques.